The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly, to a semiconductor device which includes a wiring layer and a method for manufacturing the same.
In less dense conventional semiconductor devices, metal step coverage has not been a serious problem. However, as integration density in semiconductor devices has increased, the diameters of contact holes have become significantly smaller, to half micron dimensions, while impurity-doped regions formed in the surface of the semiconductor wafer have become much shallower. Accordingly, improvement in the conventional method for forming a wiring layer, using aluminum (Al), is needed since filling contact holes of 1 .mu.m or less is difficult and the reliability of the metal wiring layer is deteriorated due to the formation of voids. The wiring method used in semiconductor devices has become very important in semiconductor manufacturing processes since it is an important factor in determining the speed, yield and reliability of the semiconductor device.
To solve such problems as void formation, caused by poor step coverage of the sputtered aluminum and high aspect ratios of contact holes, a method for filling up the contact hole with molten aluminum has been proposed. For example, Japanese Laid-open Publication No. 62-132848 (by Yukiyasu Sugano et al.), Japanese Laid-open Publication No. 63-99546 (by Shinpei Iijima et al.) and Japanese Laid-open Publication No. 62-109341 (by Misahiro Shimizu et al.) disclose melting methods. According to the above publications, the contact hole is filled by the steps of depositing aluminum or an aluminum alloy on a semiconductor wafer, heating the aluminum beyond its melting temperature, and then reflowing the liquid aluminum to fill the contact holes.
According to the above method, the semiconductor wafer has to be disposed horizontally so as to allow proper filling of the contact hole with the flowing molten aluminum. The liquid metal layer will seek a lower surface tension and, thus, may shrink or warp when this layer solidifies, thereby exposing the underlying semiconductor material. Further, the heat treatment temperature cannot be precisely controlled and, therefore, the desired results are difficult to reproduce. Moreover, the remaining areas of the metal layer besides the contact hole become rough, which causes difficulty in subsequent photolithography processes.
A metal wiring method for eliminating the poor step coverage is disclosed in U.S. Pat. No. 4,970,176 (by Tracy et al.). According to this patent, a thick metal layer having a predetermined thickness is deposited on a semiconductor wafer at a low temperature (below approximately 200.degree. C.). Then a remaining and relatively thin metal layer is deposited on the semiconductor wafer while the temperature is increased to approximately 400.degree. C. to 500.degree. C. The thus-deposited metal layer improves the step coverage of a metal layer which will be deposited later, by means of grain growth, re-crystallization and bulk diffusion.
However, even in this method, a contact hole whose diameter is 1 .mu.m or less cannot be completely filled with aluminum or an aluminum alloy.
Meanwhile, Hisako Ono et al. have shown that when the semiconductor wafer temperature is above 500.degree. C., the liquidity of Al--Si suddenly increases. A method of filling the contact hole by depositing Al--Si film at a temperature of 500-550.degree. C. was disclosed in 1990 VMIC Conference, June 11-12, pp. 76-82.
Additionally, Yoda Dakashi et al. have suggested a method for filling up the contact hole by depositing metal at a temperature of 500-550.degree. C. (European Patent Application No. 90104814.0 corresponding to Japanese Laid-open Publication No. 02-239665). According to the Yoda Dakashi method, the contact hole can be completely filled with a metal. However, there is a high probability that the Al--Si film has a strong resistance against electron migration but a weak resistance against stress migration. In addition to this, the Si included in the Al film is crystallized at the interfaces between Al--Si grains. Thus, it becomes necessary to completely remove the Al--Si film at areas other than the contact hole area, and the wiring is formed after depositing an Al--Si--Cu film.
Additionally, C. S. Park et al. (which includes the present inventor) have disclosed a method which comprises the steps of depositing an aluminum alloy at a low temperature of 100.degree. C. or below, performing a heat treatment for three minutes at a temperature of approximately 550.degree. C., i.e., a temperature below the melting point, and then completing the filling of the contact hole (see Proceedings of 1991 VMIC Conference, June 11-12, pp. 326-328). This method is included in U.S. Pat. No. 5,318,923 filed on Jun. 11, 1992 (as a continuation-in-part of U.S. patent application Ser. No. 07/585,218 entitled "A Method for Forming a Metal Layer in a Semiconductor Device," filed on Sep. 19, 1990). The aluminum deposited at low temperature is not melted during heat treatment at 550.degree. C., but migrates into the contact hole, thereby completely filling the contact hole.
According to the C. S. Park method, a 0.8 .mu.m contact hole having an aspect ratio of approximately 1.0 can be completely filled by performing heat treatment even after aluminum is deposited at a low temperature (100.degree. C. or less) to a thickness of approximately 500 .ANG.. This method does not require an etching process to be performed, as in the Yoda Dakashi method. Because of these advantages, the C. S. Park method for filling the contact hole is attracting much interest in the relevant fields.
Pure Al has been used for forming metal wiring layers in the early development of semiconductor devices. However, Al--1% Si, i.e., aluminum over-saturated with silicon, has now been widely used as a metal wiring layer material since the Al layer absorbs silicon atoms from the silicon substrate and generates junction spiking as the temperature increases in the sintering step.
However, when the semiconductor device wiring is formed using Al-1% Si, silicon from the Al film crystallizes during heat treatment, performed at a temperature of approximately 450.degree. C. or higher, thereby forming Si precipitates. The silicon grain formation is accomplished by an epitaxial growth in the contact hole, to thereby form a Si-nodule. As a result, the Si precipitate or Si-nodule increases the wiring resistance or the contact resistance.
It is presently known that a diffusion barrier layer can be formed between the wiring layer and the silicon wafer or an insulating layer, so as to prevent Al spiking, Si precipitates and Si-nodule formation caused by the above-mentioned reaction between the metal wiring layer and the silicon wafer. For example, U.S. Pat. No. 4,897,709 (by Yokoyama et al.) describes a method for forming a nitride titanium film as a diffusion barrier layer on the inner walls of the contact hole. Additionally, in Japanese Laid-open Publication No. 61-183942, a method is described for forming a barrier layer which comprises the steps of forming a refractory metal layer by depositing a metal such as Mo, W, Ti or Ta, forming a titanium nitride layer on the refractory metal layer and heat-treating the double layer which consists of the refractory metal layer and the nitride titanium layer to thereby form a refractory metal silicide layer consisting of thermally stable compounds at the intersurface of the refractory metal layer and semiconductor substrate. Thus, the barrier characteristic is improved. This heat treatment of the diffusion barrier layer is performed by an annealing process under a nitrogen atmosphere. When the diffusion barrier layer does not undergo the annealing process, junction spiking occurs in a subsequent sintering step after Al sputtering or while sputtering Al or an Al alloy at a temperature about 450.degree. C., which is undesirable.
Additionally, Hagita Masafumi has suggested a method wherein a TiN layer, as a barrier layer, is heat-treated and then the barrier layer is implanted with O.sub.2 or silicon in order to improve the wettability between the barrier metal and the Al wiring and to improve the quality and yield of the wiring (Japanese Laid-open Publication No. 2-26052).
Additionally, a method for improving a barrier characteristic upon forming a diffusion barrier layer is known. This method comprises a step of forming a TiN layer, heat-treating, and then forming a TiN layer again.
Besides the method for preventing Al spiking or Si precipitate crystallization by improving the characteristics of a diffusion barrier layer, as described above, a method for preventing Al spiking or the formation Si precipitates by forming a composite layer having various compositions, such as an Al wiring layers, has also been suggested.
For example, a method for preventing Si-precipitates in a sintering process when a wiring layer is formed is disclosed in Japanese Laid-open Publication No. 2-159065 (by Michiichi Masmoto). This method comprises the steps of forming an Al--Si film and then forming a pure Al layer thereon, thereby preventing Si-precipitates in the sintering process. Further, U.S. Pat. No. 5,266,521 (filed on Jan. 31, 1992) and U.S. Pat. No. 5,355,020 (filed on Jul. 8, 1992) by S. I. Lee (the present inventor) et al. disclose a method for forming a composite layer so as to prevent the crystallization of Si precipitates generated when the contact hole is filled by depositing Al at a low temperature and heat-treating at a high temperature below the melting point according to the C. S. Park et al. method. According to the method described in U.S. Pat. No. 5,266,521, pure Al is deposited at a low temperature to a thickness of approximately one third of a predetermined thickness of a wiring layer so as to form a first metal layer. Then, the first metal layer is heat-treated at a temperature of approximately 550.degree. C., to thereby fill the contact hole. Then, an Al alloy which includes a Si component is deposited at a temperature of approximately 350.degree. C. so as to form a second metal layer. Thus, by forming a composite layer, the first metal layer which does not include the Si component absorbs the silicon from the second metal layer in the subsequent sintering process, thereby preventing the crystallization of Si precipitates. According to the method of U.S. Pat. No. 5,355,020, an Al alloy including a Si component is first deposited and then pure Al or an Al alloy is deposited, thereby forming a composite layer. Then, this layer is heat-treated so as to fill the contact hole. Then, an Al alloy without Si is additionally deposited so as to have a predetermined thickness, which then is patterned, to thereby complete a wiring layer.
Generally, in order to form a metal layer after forming a diffusion barrier layer, the wafer is exposed to the atmosphere since the wafer should be transferred to sputtering equipment to form the metal layer.
At this time, oxidation occurs at the interfaces of the grains or on the surface portion of the diffusion barrier layer, and the mobility of aluminum atoms on the oxidized diffusion barrier layer is decreased. When an Al--1% Si--0.5% Cu alloy is deposited to a thickness of 6,000 .ANG. at room temperature, the formed grains are small, i.e. approximately 0.2 .mu.m.
Meanwhile, large grains of up to approximately 1 .mu.m are formed on the diffusion barrier layer unexposed to the atmosphere. Aluminum reacts with the diffusion barrier layer during a heat-treating step at a high temperature or when depositing an Al film by sputtering at a high temperature, to thereby make the surface of the Al film very rough and deteriorate surface reflectivity thereof. As a result, subsequent photolithography processes are difficult to perform.
In general, a titanium nitride (TiN) layer or TiW (or TiW(N)) layer is used as the diffusion barrier layer. Such layers have micro-structured defects or grain boundaries which cannot prevent silicon or Al diffusion at the grain boundary when forming a thin film of the diffusion barrier layer. A method for blocking a diffusion path in the grain boundary according to an "oxygen stuffing" method has been suggested. When the diffusion barrier layer is exposed to a N.sub.2 annealing process or to the atmosphere, a small amount of oxygen is mixed into the barrier layer, to thereby increase a diffusion barrier effect. This is called a "stuffing effect."
Generally, when TiN is deposited and exposed to the atmosphere, a stuffing effect occurs due to oxygen in the atmosphere. The method of the Hagita patent also oxygenates the surface of the diffusion barrier layer, thereby improving the characteristics of the barrier metal.
However, contact resistance can be increased when Ti or TiN is deposited to form a barrier layer which is then exposed to the atmosphere, when TiN is deposited while introducing the oxygen, or when the barrier layer is annealed under a nitrogen atmosphere wherein oxygen is introduced.
The barrier characteristics of TiN film can be changed depending on such conditions as the exposure time to the atmosphere, the amount of oxygen introduced during deposition, the amount of oxygen introduced during annealing, and the annealing temperature. Annealing the barrier metal is best performed at a temperature of approximately 450.degree. C. under a N.sub.2 atmosphere for 30 to 60 minutes.
FIG. 1 is a cross-sectional view showing an oxide layer formed on a surface of the diffusion barrier layer when the vacuum is broken after forming the diffusion barrier layer. FIG. 2 is a cross-sectional view showing the oxide layer formed on the surface of the diffusion barrier layer after its formation and the N.sub.2 annealing thereof, showing an improved diffusion barrier characteristic.
FIG. 3 is a sectional view showing a diffusion barrier layer obtained by forming a first diffusion barrier layer and then forming a second diffusion barrier layer on the first diffusion barrier layer after ion-implanting the first diffusion barrier layer or nitrogen annealing the first diffusion barrier layer. Referring to FIG. 3, as a middle layer, an amorphous layer formed by ion-implantation or an oxide layer formed by nitrogen annealing exists between the first diffusion barrier layer and the second diffusion barrier layer.
After the diffusion barrier layer is formed, and when the Al wiring layer is formed according to the C. S. Park method or the high-temperature sputtering method, the diffusion barrier layer is exposed to the atmosphere. Therefore, an oxide exists on the surface of a diffusion barrier layer and on the grain boundary thereof, thereby deteriorating the wettability between the diffusion barrier layer and the Al wiring layer. As a result, the size of the grain formed in the early step of the deposition becomes smaller, and the reliability for the wiring layer is lowered due to a poor profile of the deposited Al, void formation while filling the contact hole, or a poor profile of the Al layer during heat treatment.
FIGS. 4, 5 and 6 show a poor wiring layer which can be generated when Al is deposited on a conventional diffusion barrier layer to form a metal layer and then the metal layer is heat-treated to fill the contact hole.
Referring to FIGS. 4-6, reference numeral 1 denotes a semiconductor substrate, reference numeral 2 denotes an impurity doped region, reference numeral 3 denotes an insulating film (BPSG film), reference numeral 4 denotes a diffusion barrier layer, and reference numeral 6 indicates an Al alloy metal layer. FIG. 4 shows a discontinuity 7 of the Al alloy metal layer generated on sidewalls of the contact hole due to poor wettability between the diffusion barrier layer 4 and the Al alloy metal layer 6 when performing high-temperature sputtering or Al deposition. FIG. 5 shows a void 8 which exists in the contact hole when the Al alloy metal layer 6 is deposited and then heat-treated in a vacuum or when the contact hole is filled according to the high-temperature sputtering method. FIG. 6 shows a poor profile 9 of the Al alloy metal layer 6 generated when Al is sputtered at a high temperature or when the Al alloy metal layer 6 is heat-treated in a vacuum after Al deposition.
Hiroshi Nishimura et al. have suggested that a via hole having a diameter of 0.5 .mu.m and an aspect ratio of 1.6 can be filled by depositing Ti prior to Al sputtering and by successively performing high-temperature sputtering of Al at a temperature of approximately 500.degree. C. ("Reliable Submicron Vias Using Aluminum Alloy High Temperature Sputter Filling" pp. 170-176, 1991 VMIC Conference). According to Hiroshi et al., the filling of the contact hole is caused by the reaction between Al and Ti. However, when Al.sub.3 Ti is formed, the solid solubility of silicon increases to approximately 15% by weight at a temperature of 450.degree. C. when sintering is performed in a subsequent process. Accordingly, when Al.sub.3 Ti is formed in the contact hole, the possibility of generating Al spiking increases by the reaction of the Al layer with the substrate due to the reaction between Al.sub.3 Ti and Si. Further, the Al surface becomes very rough as the Al and Ti react, when the vacuum heat-treatment is performed according to the C. S. Park method after deposition process, or when sputtering Al at a high temperature, thereby lowering reflectivity and creating difficulties in subsequent photolithography processes.
Additionally, when depositing a metal including no Si components in order to prevent the crystallization of Si precipitates, Al spiking occurs by the reaction between the Al and the substrate where a poor diffusion barrier layer exists. FIG. 7 is a sectional view showing Al spiking generated from the conventional techniques. Referring to FIG. 7, reference numeral 1 denotes a semiconductor substrate, reference numeral 2 indicates an impurity doped region, reference numeral 3 denotes an insulating film (BPSG film), reference numeral 4 denotes a diffusion barrier layer, reference numeral 6 denotes an Al alloy metal layer and reference numeral 10 denotes Al spiking.
It is desirable to form an oxide on the surface of the diffusion barrier layer and in the grain boundary thereof, in order to improve the characteristics of the diffusion barrier layer in the contact hole. However, this oxide may deteriorate the wettability of the diffusion barrier layer and the Al so that a void may be formed in the contact hole, or a metal layer which has a poor profile during heat treatment may be formed, which thereby deteriorates the reliability of the wiring layer of the semiconductor device.
In addition, when the composite layer is formed according to the methods disclosed in the abovementioned U.S. Pat. Nos. 5,266,521 or 5,355,020, the Al depositing process is complicated. As a result, the throughput decreases or the conditions for forming a diffusion barrier layer become complicated. Therefore, the processing margin is narrowed, which is undesirable.
Dipankar Pramanik and Vivek Jain have announced their results of an experiment wherein an Al--1% Cu alloy is deposited on various kinds of underlayers at a temperature of 170.degree. C. (see "Effect of Underlayers on Sputtered Aluminum Grain Structure and its Correlation with Step Coverage in Submicron Vias," 1990 VMIC Conference, June 12-13, pp. 332-334). Dipankar et al. teach that the grain size of Al generated during the deposition varies depending on the kind of underlayer and the best step coverage can be obtained from a TiW film having the largest grain formation. This step coverage of Al is closely related to the size of the Al grains formed during deposition. That is, the larger the Al grain formed during deposition, the better the step coverage for Al layer contact holes or via hole. In addition, larger Al grains are formed during the deposition process when the wettability between the Al grains and the underlayer is better.